The present invention relates to apparatus, such as radars and sonars, which store received echo signals defined in a polar coordinate system into a video memory having an array of memory elements arranged in the form of a Cartesian coordinate system to cover all directions in a horizontal plane, and then present the stored data on a raster-scan display screen. More particularly, the invention is concerned with radar and like apparatus which have two video memories, one for storing a current image and the other for storing a past image, and display the image data stored in the two video memories with the past and current images superimposed on each other.
FIG. 1 is a block diagram of a conventional radar apparatus.
While rotating at a specific speed in a horizontal plane, a radar antenna 1 transmits pulses of radio waves at a specific pulse repetition rate and receives echoes of radio waves reflected by targets. A receiver circuit 2 detects and amplifies signals received by the radar antenna 1. An analog-to-digital (A/D) converter 3 converts an analog signal obtained by the receiver circuit 2 into a digital signal. A primary memory 4 stores A/D-converted data for one sweep in real time, and is used as a buffer for writing the one-sweep data into a video memory in a succeeding stage until the stored data is overwritten by new one-sweep data resulting from a next transmission and obtained.
A selector 11 switches a clock input to the primary memory 4 between a write clock necessary when writing data into the primary memory 4 in real time and a read clock necessary when transferring the data to the video memory.
A coordinate converter 12 generates addresses representative of pixels of the video memory arranged in a Cartesian coordinate system successively from the center of the system outward, a start address corresponding to coordinates of the center, based on antenna direction xcex8 referenced to the heading of a ship and a location in the primary memory 4 from which a signal is read out, for example. Specifically, the coordinate converter 12 is constructed of hardware which performs operations expressed by the following equations:
X=Xs+rxc2x7sin xcex8
Y=Ys+rxc2x7cos xcex8
where
X and Y are coordinates of an address representative of a pixel in the video memory;
Xs and Ys are coordinates of the center address;
r is the distance from the center to the pixel; and
xcex8 is the direction of a pixel for coordinate conversion.
In the apparatus described above, received data are distributed densely around the sweep center and sparsely in peripheral areas from a geometrical point of view and, therefore, the nearer to the sweep center, the larger the number of data corresponding to the same addresses of the video memory. There can be a case where more than one successive data corresponds to the same address of the video memory while a row of received data is being written into the video memory. When a plurality of different data correspond to the same pixel as seen in the above example, a problem would arise if a simple overwriting method is used. This is because if data written in one pixel is simply overwritten by succeeding data assigned to the same pixel, only the last written data would be left, invalidating the previously written data. One conventional approach taken to overcome this problem is, for example, a maximum data sampling method, in which data having a maximum value among all received data assigned to the same address is written therein. In this maximum data sampling method, it is determined whether data is being written for the first time, or the second or more time, in each address of the video memory. If data is being written for the first time, most recently received data is written by a write data generator 5. If, however, data is being written for the second or more time, the write data generator 5 compares the most recently received data with the previously written data and writes the data having a larger value. Consequently, data having the maximum value among all received data assigned to the same pixel is obtained.
A current-image video memory 7 is a memory for storing the latest target image, wherein the most recently received data for one frame is stored via the write data generator 5. A past-image video memory 8 stores a series of target images by accumulatively storing the received data. This past-image video memory 8, when employed in a shipborne radar, for example, allows an operator to see the track of a target ship by superimposing the previously stored image on the latest image. Provision of this memory 8 helps to recognize surrounding situations and enhance navigational safety. Also, when the past-image video memory 8 is employed in a sonar, it becomes possible to recognize the movement of fish schools.
In the radar apparatus thus constructed, data to be written into the past-image video memory 8 is generated by a track data generator 6. When the received data is equal to or higher than a specific level, the track data generator 6 determines that there is track data and writes the track data in a corresponding address of the video memory 8. In this process, a logical sum of previously written track data and the new track data is taken as current write data, thus allowing past target images to be accumulated. In actual applications, however, simply accumulating the past target images would result in a cluttered picture because unwanted noise becomes accumulated with the lapse of time. To avoid this inconvenience, a practical processing technique is used, such as to automatically erase past target tracks which have been stored in the past-image video memory 8 for over a specific period of time.
Each of the video memories 7, 8 has a storage capacity sufficient to store the data received at least during one antenna rotation. An unillustrated display controller reads out data contents of the video memories 7, 8 at a high speed in synchronization with scanning of a cathode ray tube (CRT) 10. A mixer 9 mixes the latest target image and the past target images in a manner that the two kinds of the images can be discriminated therebetween by monochrome shading or color gradations and outputs the images to the CRT 10 for on-screen presentation.
When the above-described conventional apparatus is switched from one range (detection range) to another to change display range scale, the target images stored on the previously used range are unusable because they do not fit to the new range scale. It is therefore necessary to once erase all the previously stored target images and newly store target images on the new range scale. Thus, there has been a problem that the operator could not recognize movements of target ships for some time after switching the range scale. If the past target images is preserved even when the range scale has been switched, the preserved past target images might be reused when the apparatus is switched back to the original range. The preserved past target images would be unusable, however, if a newly selected range differs even slightly from the original range.
To solve these problems, it might be possible to prepare a plurality of past-image video memories and simultaneously store target tracks corresponding to a plurality of ranges. This, however, is economically difficult to put into practical use.
Accordingly, it is an object of the invention to provide radar and like apparatus, in which when the range scale has been switched, past target images which have been stored are converted to fit to a new range scale such that the past target images can be displayed in continuity on the new range scale without erasing the previously stored past target images.
Radar and like apparatus of the present invention comprise a current-image video memory for storing most recently received data, a past-image video memory for accumulatively storing previously received data, an indicator for superimposing and displaying the data stored in both the current-image video memory and the past-image video memory, and a controller for transferring the data stored in the past-image video memory to a buffer in a first step and re-transferring the data stored in the buffer back to the past-image video memory in a second step when display range scale has been changed, wherein address shift data K specifying the amount of address shift per transfer cycle in the buffer and the past-image video memory for said data transfer and re-transfer processes is changed in accordance with the range scale.
The current-image video memory may be used also as the buffer.
The controller may perform such a control operation that clears data content of the past-image video memory, which is a transfer source, at the same time when the data is transferred in the first step.
In the event of range switching, that is, when the range scale has been changed, the controller transfers the data stored in the past-image video memory to the buffer in the first step and re-transfers the data stored in the buffer back to the past-image video memory in the second step. In such data transfer and re-transfer processes, the controller changes the address shift data K specifying the amount of address shift per transfer cycle in the buffer and the past-image video memory in accordance with the range scale. With this arrangement, the data stored in the past-image video memory is scaled down or up in the data transfer and re-transfer processes performed when the range scale has been changed, so that an image conforming to a newly selected range scale is stored in the past-image video memory. As a consequence, the indicator continues to display a past target image even after the range scale has been changed. Although it is possible to provide the buffer as an independent element, the current-image video memory whose data content is entirely rewritten after the range scale has been changed may be used as the buffer. Furthermore, if the data content of the past-image video memory, which is the transfer source, is cleared at the same time when the data is transferred in the first step, a process for clearing the past-image video memory at a later time becomes unnecessary, and this make it possible to reduce the time required for data transfer.
Also, the controller may set the address shift data K (1) to satisfy K=1 for a transfer source and K=1/N for a transfer destination when scaling an on-screen image by a factor of 1/N, and (2) to satisfy K=1/N for the transfer source and K=1 for the transfer destination when scaling the on-screen image by a factor of N.
Since 0 less than K less than =1 when K is set as described above, this approach facilitates the setting of K.
Furthermore, when executing a scale-down operation in which multiple memory pixels of a transfer source correspond to one memory pixel of a transfer destination, the controller may cause data having a maximum value among data in the multiple memory pixels of the transfer source to be stored in the one memory pixel of the transfer destination.
While the aforementioned maximum data sampling operation is an operation to be performed in coordinate conversion, it becomes possible to prevent important data from being erased by executing the maximum data sampling operation in the data transfer process performed in the event of range switching.